Most antennas that are designed for hand-held and portable communications devices are external, attachable extrusions. For example, many cellular telephones use a form of monopole or dipole antenna that protrudes from the telephone case which houses the rest of the communications electronics.
While an external antenna may be acceptable for devices such as hand-held cellular telephones, it is not as desirable in other applications, such as a wireless communications device that is adapted to be used with a laptop computer or the like. For example, if the computer is located in a confined space, an external antenna which extends therefrom may prove to be cumbersome and provide an unnecessary constraint on the locations at which the computer can be placed. In addition, because of its external nature, the antenna could be easily broken as the computer is moved. Furthermore, since the antenna is accessible by the user, the possibility exists that it could be modified, and thereby alter the communications parameters to values other than those for which the communications device was designed.
Accordingly, it is desirable to provide an antenna arrangement that can be internally housed within the structure of a communications device. By building the antenna into a case that houses the communications electronics, for example, several levels of protection are provided. First, the antenna is not subject to breakage or illegal access, thereby increasing the reliability and integrity of the communications system. In addition, an internally located antenna has reduced proximity to human operators, thereby providing an increased safety margin relative to electromagnetic fields and other forms of radiation associated with antennas.
While internally mounted antennas provide the foregoing advantages, they also have attendant constraints. First and foremost among these is the antenna's size. Typically, a monopole antenna has a height equal to about one-fourth of the wavelength for the carrier frequency over which communications are carried out. Even at a relatively high frequency of 2.4-2.5 GHz, the one-fourth wavelength height of the antenna is approximately 1.2 inches. In a structure such as a laptop computer, or an attachment for such a computer, the internal dimensions of the case may not provide sufficient space for an antenna of this height. Accordingly, when an antenna is to be incorporated into a small structure, some physical modification of the antenna is required, to accommodate the available space. Such a modification typically alters the radiation transmission and reception characteristics of the antenna, however, which may prove to be undesirable.